Although this game is 2d, basic, and stickman-ish (think stick-death); it is very entertaining. The link I'm posting based on dirt bike supercross, the creator has another map that is based for the mtb or bmx bike. This creator has two track maps, while I believe there are thousands out there. Try it out, and post up with your thoughts... good or bad. Remember, try both of this users tracks, and also there are around 5 or 6 vehicles to choose from. I usually just use the mtb.

Here are both links (I wish we could embed links)...

Here's the mtb one...
Rocky Trail to Warehouse
http://trackmill.com/free-rider-2/Tracks/697020

Las Vegas Supercross...
http://trackmill.com/free-rider-2/Tracks/699452